The mechanism of hydrogen embrittlement
There are various schools of thought on the mechanism of hydrogen embrittlement: hydrogen adsorption theory, pressure expansion theory, hydrogen-dislocation interaction theory, lattice embrittlement theory, hydride or hydrogen-rich phase precipitation theory, hydrogen-assisted fracture (HAC) theory, etc. . Each school has a certain experimental basis, which can explain some hydrogen embrittlement phenomena.
In 1952, N.T.Petch and P.Stabk proposed the theory of hydrogen adsorption. The theory holds that the surface energy r of the metal decreases due to the adsorption of hydrogen at the tip of the crack. According to Griffith theory, the fracture strength σc of the metal is proportional to r1/2. With the decrease of the surface energy r, the fracture strength σc is also decrease, thus causing embrittlement of the material. N.T. Page et al. believed that the surface energy of the crack was reduced due to the adsorption of argon atoms. When the crack tip is in the cathodic state, a large number of hydrogen atoms are generated due to the cathodic reaction. According to the fracture mechanics, the surface at the crack tip with high stress will effectively promote the surface adsorption of hydrogen atoms.
The theory of hydrogen pressure expansion was proposed by C. ZapHe in 1947. The theory of hydrogen pressure believes that hydrogen segregates to defects such as pores, holes, mosaic structures, and dislocations inside the material under the action of stress, and combines into hydrogen molecules, causing a large pressure in the micropores (up to 9.8l×10^5MPa). The superposition of internal pressure and the internal stress or external stress of the material will cause the crack to expand, resulting in cracking. Since high pressure is controlled by the diffusion rate of hydrogen atoms, crack propagation is determined by the ability of hydrogen to diffuse in the material. At lower temperatures, hydrogen embrittlement slows or even stops. The hydrogen pressure expansion theory can explain the formation mechanism of fisheye white spots. When the material is subjected to enough tensile stress, microcracks will be generated at the interface between pores and matrix, or at the interface between inclusion and matrix, or at the inclusion itself. Hydrogen atoms will segregate to the crack and combine into hydrogen molecules, resulting in huge pressure. Under the action of external tensile stress, it explodes into a local brittle fracture area, and the fracture shows fish-eye-shaped white spots with pores or inclusions as the core.
According to the theory of interaction between hydrogen and dislocations. The hydrogen atoms gathered in the three-way stress zone of the notch or crack front interact with the dislocations, so that the dislocations are pinned and cannot move freely, resulting in local hardening. Under the action of external force, the matrix cannot relax the stress through plastic deformation. Energy can only be released in the form of cracks. After the crack enters the hydrogen-deficient region, the dislocation movement is restored to freedom, and the stress can be relaxed through plastic deformation. The crack stops growing. When the hydrogen concentration at the front of the crack reaches a critical value, the crack grows again until it finally breaks.
In 1960, A.R. Troiano proposed the theory of lattice embrittlement. He believes that the high concentration of solid solution oxygen segregated on grain boundaries and phase boundaries reduces the bonding force between atoms in the metal lattice. When the tensile stress in the local area is greater than the interatomic bonding force reduced by hydrogen, the interatomic bonding force is destroyed, and brittle fracture occurs.
In 1969, D.G. Westlaka et al. studied the embrittlement caused by Zr-H, Nb-H, V-H and other alloy hydrides, and proposed the theory of hydride or hydrogen-rich phase precipitation. For metal materials containing hydrogen, only when the hydride or hydrogen-rich precipitate phase is precipitated, the plasticity of the material is reduced and embrittlement occurs.
In 1972, C.D. Beachem proposed the hydrogen-assisted fracture theory. He believes that the degree of plastic deformation at the crack front depends on the stress intensity factor K and the concentration of hydrogen. When K is high enough and the hydrogen concentration is high enough, there is a large plastic deformation zone at the front of the crack. If K is small, quasi-cleavage or intergranular fracture will occur.